Transmission lines in conventional integrated circuit packages are designed to match a desired impedance level, generally identified by a resistive value. For example, integrated circuits have been produced with transmission lines having impedance values that range from tens of Ohms to about a hundred Ohms depending on application specific needs to match circuits and devices that are not part of the integrated circuit.
A transmission line is a pair of parallel conductors exhibiting certain characteristics due to distributed capacitance and inductance along the length of the transmission line. Every transmission line possesses a characteristic impedance, usually designated as Z0. Z0 is the ratio of electrical potential, E, to current, I, at every point along the transmission line. If a load equal to the characteristic impedance is placed at the output end of any length of a transmission line, the same impedance will appear at the input terminals of the line. The characteristic impedance determines the amount of current that can flow when a given voltage is applied to an infinitely long transmission line. Even though characteristic impedance is quantified by a measure of electrical resistance in Ohms, the characteristic impedance of a transmission line is entirely different from the leakage resistance of the dielectric separating the two conductors and the metallic resistance of the wires themselves. Characteristic impedance is a function of the capacitance and inductance distributed along the line's length. Thus, characteristic impedance would exist even if the dielectric was perfect and the metal used in the transmission lines had no series resistance.
It is desirable to match the characteristic impedance of transmission lines to other circuits, elements and external devices because the most efficient transfer of electrical energy takes place when the characteristic impedance of a first circuit, element or external device matches the characteristic impedance of the transmission line. When the characteristic impedance of the transmission line matches the impedance of a coupled circuit, element, or external device, energy will traverse the interface without reflection. Thus, no signal power will be lost.
An example simplified metallization layer of an integrated circuit with conventional transmission lines is illustrated in FIG. 1. The arrangement in FIG. 1 is a top plan view that illustrates the relative locations of four conductors along a portion of a metallization layer 10. The metallization layer 10 includes a left-side ground (AGND) conductor 12 and a right-side ground (AGND) conductor 14 along with a transmission line 13 and a transmission line 15. The transmission line 13 and the transmission line 15 each have characteristic impedances of 100 Ohms. That is, a circuit, element, or external device coupled to the transmission line 13 or the transmission line 15 will encounter a resistance of 100 Ohms.
Microwave theory indicates that varying the physical dimensions of a transmission line along its length will vary the characteristic impedance along the transmission line. However, the transmission line must have a length that is an integer multiple of ¼ the wavelength of the signal frequency and must be coupled to a fixed load. Both conditions, for reasons explained in greater detail below, are not present when communicating high frequency signals in an integrated circuit coupled to a load with varying impedance.
The implementation of a transmission line, such as the transmission line 13 and the transmission line 15 introduces a number of challenges. First, load impedances are not constant over frequency and temperature. Nearly all loads can be modeled as a resistor-capacitor (RC) network or a circuit of resistors and capacitors. The resistance value of a resistor varies with temperature. Similarly, the capacitance value of a capacitor varies with frequency. Consequently, when a transmission line is intended to match a desired impedance, signal return loss will vary with environment and circuit conditions. Return loss suffers significantly when the transmission line 13 or the transmission line 15 carry a high-frequency signal as the actual impedance will be lower than the desired impedance.
Furthermore, transmission lines implemented in an integrated circuit are not infinite in length. Not only are the transmission lines in an integrated circuit finite in length, but signals up to about 10 GHz will generally not have a length that is an integer multiple of a quarter of the wavelength of the signal. For example, a 1 GHz signal has a wavelength of 30 centimeters. Whereas a 10 GHz signal has a wavelength of 3.0 centimeters. The length of an integrated circuit transmission line will rarely be a multiple of ¼ of 3.0 centimeters. Accordingly, for at least these reasons, the transmission line itself will be responsible for some return loss due to reflections caused by the length of the transmission line.
Moreover, the various sets of conductors forming the multiple transmission lines in an integrated circuit will vary in length from transmission line to transmission line as not every path through the integrated circuit will have a similar length.
With the development of increasingly faster integrated circuits there is a corresponding increase in the demands made upon integrated circuits and integrated circuit assemblies for efficiency and frequency response. For example, the Commission Electrotechnique Internationale (CEI or International Electrotechnical Commission) has published a specification for return loss over frequency that limits the magnitude of acceptable return loss for integrated circuits, sub-assemblies and completed devices in which the circuits are used.
The plots of FIG. 2 and FIG. 3 reveal example time domain reflectometry data and return loss data, respectively for two example combinations. FIG. 2 illustrates the time-domain reflectometry (TDR) data for a first combination of a printed-circuit board and a conventional 100 Ohm package, as well as a second combination of a die, a printed-circuit board and a 100 Ohm transmission line. The plot shows time in nanoseconds (ns) from a reference time of 0.0 nanoseconds at the left most side of the plot to 0.8 nanosecond at the right most side of the plot vs. impedance in Ohms over a range of 80 to 100 moving up the plot.
As indicated by the trace 20 the first combination of the printed circuit board and the conventional 100 Ohm package has a TDR of approximately 100 Ohms from 0.0 nanoseconds to nearly 0.225 nanoseconds. From approximately 0.225 nanoseconds to about 0.375 nanoseconds, the TDR falls to a local minimum of approximately 87 Ohms. Thereafter, from between approximately 0.375 nanoseconds and 0.550 nanoseconds, the TDR rises until the TDR reaches about 100 Ohms. From about 0.550 nanoseconds to approximately 1.0 nanosecond, the TDR falls steadily about 2 to 3 Ohms from about 100 Ohms to about 97 Ohms. The trace 24, representing the second combination of the die, the printed-circuit board and the 100 Ohm transmission line, follows the trace 20 until just after 0.4 nanoseconds, where a local maximum value of about 90 Ohms is reached at approximately 0.45 nanoseconds. From the local maximum at about 0.45 nanoseconds the TDR falls until a second minimum value of about 82 Ohms is reached at approximately 0.575 nanoseconds. From 0.575 nanoseconds to about 0.8 nanoseconds the TDR rises from the local minimum of about 82 Ohms to about 96 Ohms. The TDR for the second combination remains at approximately 96 Ohms from about 0.8 nanoseconds to 1.0 nanosecond.
FIG. 3 illustrates the return loss in decibels (dB) vs. frequency in gigahertz (GHz) for the first combination (printed-circuit board and a conventional 100 Ohm package) and the second combination (the die, the printed-circuit board and the conventional 100 Ohm transmission line). Trace 30 represents a specified or minimum return loss as published by the CEI. As shown by trace 34, return loss increases from about −45 dB at 0 GHz to about −15 dB at 1.4 GHz. Thereafter, the return loss falls from the local maximum of about −15 dB to about −30 dB at approximately 2.4 GHz. From the local minimum at approximately 2.4 GHz, the return loss increases until about 4.75 GHz. From just under the local maximum level at 4.75 GHz to about 5.2 GHz, the return loss overlaps the specified return loss. That is, the return loss is out of specification for signals of about 4.75 GHz to about 5.2 GHz.
There are 3 possible ways to improve an impedance mismatch, all of which are called “impedance matching.” The first way to improve an impedance mismatch is to present an apparent load to the source of Rload=Rsource* (complex conjugate matching). The second way to improve an impedance mismatch is to present an apparent load of Rload=Rline (complex impedance matching), to avoid echoes. Given a transmission line source with fixed-source impedance, this “reflectionless” impedance matching at the end of the transmission line is a possible way to avoid reflecting echoes back to the transmission line. The third way to improve an impedance mismatch uses devices intended to present an apparent source resistance as close to zero as possible, or presents an apparent source voltage that is as high in magnitude as possible.
Transformers and combinations of resistors, inductors and capacitors have been used to match electrical impedances. These impedance matching devices are optimized for different applications, and are called baluns, antenna tuners, acoustic horns, matching networks, and terminators.
Transformers can match the impedances of circuits with different impedances. A transformer converts alternating current at one voltage to the same waveform at another voltage. The power input to the transformer and output from the transformer is the same absent conversion losses in the transformer. The winding with the lower voltage is at low impedance, because this winding has the lower number of turns, and the winding with the higher voltage is at a higher impedance as it has more turns in its coil. Resistive impedance matches are easiest to design and are used to transfer low-power signals, such as unamplified audio or radio-frequency signals in a radio receiver. Almost all digital circuits use resistive impedance matching which is usually built into the structure of the switching element.
Transformers and combinations of resistors, inductors and capacitors, if used to match the impedance of transmission lines in an integrated circuit, would require additional circuit area and additional power. Therefore, it would be desirable to reduce return loss such that integrated circuits and printed circuit board combinations, as well as integrated circuit assemblies that include a combination of an integrated circuit, a printed circuit board and a die could exceed a specified return loss over a broader range of operating frequencies absent transformers and additional circuitry.